I was fortunate to own a Mesa M6 at the same time as my Mesa D-180 and 400+ and experimented with the M6 preamp into the power sections of both of the tube amps (and vice versa)- I found the M6 pre into the D-180 preamp to be the best of both worlds personally! A clarity and depth to the tone that wasn't inherent in the 400+ preamp, but still retaining that powerful midrange voicing. I mentioned this experiment in one of the early M6 threads and another TB'er PM'ed me with the info that Mesa was actually toying with this combination (I'm glad to see they followed through). Having the selectable output power will likely give you plenty of control as far as gainy tones are concerned (drop down to 150 watts for the D-180 grind). I much preferred the sound of my 400+ with the amp cranked than I did with just the preamp cranked and my experimentation with the M6 pre into the 400+ power amp was honestly even better for the short time I used it (and certainly easier to dial in for me- and this is coming from someone who holds the 400+ as just about the "holy grail" of bass amps!).
As far as 12 6L6s vs. 6 6550s- I ran my 400+ for years with no problems whatsoever with tubes or heat, and I can't say the same for either of the SVTs I've owned (each for shorter periods of time than the 400+ for that matter, not ragging on the SVTs though as I loved both of them as well). The 400+ was also lighter than either of those amps, even in a rack case! I definitely don't understand their reasoning for it (unless it is simply that they don't care to redesign a power section and have to deal with road-testing a new design), but it doesn't really bother me having been plenty happy with that design in the past.
As far as 12 6L6s vs. 6 6550s- I ran my 400+ for years with no problems whatsoever with tubes or heat, and I can't say the same for either of the SVTs I've owned (each for shorter periods of time than the 400+ for that matter, not ragging on the SVTs though as I loved both of them as well). The 400+ was also lighter than either of those amps, even in a rack case! I definitely don't understand their reasoning for it (unless it is simply that they don't care to redesign a power section and have to deal with road-testing a new design), but it doesn't really bother me having been plenty happy with that design in the past.
I wanted and have been waiting for a 400 watter from Mesa!